There are powerful reasons to believe Britons will ultimately vote to stay in the EU, but the alternative can’t be dismissed as a minor risk.

A British referendum on whether to remain a member of the European Union is the single biggest “known unknown” hanging over the European economy. The vote now seems almost certain to take place this year. Prime Minister David Cameron hopes to achieve a deal at a summit in February on the changes to the relationship that he thinks are necessary to persuade Britons to back continued membership. EU officials say the parameters of the deal are already hammered out. That should pave the way for a referendum either just before or immediately after the summer.